The present invention relates to a process for preparing large-sized polymer particles and more particularly to a process for preparing large-sized polymer particles having a particle diameter falling within a range of about 0.1 to 500 .mu.m by seeded polymerization.
Polymer particles, particularly monodisperse polymer particles having a particle diameter range of about 0.1 to 500 .mu.m are demanded in many fields, but are generally quite difficult to prepare. Emulsion polymerization can produce monodisperse polymer particles relatively easily, but only small-sized particles having a diameter of less than 1 .mu.m. It is said that even under specific conditions only polymer particles having a diameter of 3 .mu.m at most can be prepared by emulsion polymerization. On the other hand, suspension polymerization can produce relatively large polymer particles having a diameter of 1 to 200 .mu.m. The particle diameter distribution of these polymer particles, however, is broad; monodisperse polymer particles are difficult to prepare by suspension polymerization. Therefore, in preparing monodisperse, large-sized polymer particles, it is necessary to classify polymer particles obtained by suspension polymerization. This method, however, has disadvantages in that the number of steps is increased, making the process complicated, and the yield is low.
A method of preparing relatively large-sized, monodisperse polymer particles is disclosed in Japanese Patent Application Laid-Open Nos. 97582/1979 and 126288/1979.
The method of Japanese Patent Application Laid-Open No. 97582/1979 is such that a chain transfer agent is added during emulsion polymerization to thereby prepare polymers having a much lower molecular weight than the usual polymer latex, and with these polymers as seed particles, a slightly water-soluble unsaturated monomer is made absorbed on the seed particles and then polymerized. In this method, however, when commonly used oil-soluble or water-soluble polymerization initiators are used, problems such as aggregation and formation of new particles arise. Thus large-sized, monodisperse polymer particles are difficult to prepare with high reliability and in high yield by the above method.
In the method disclosed in Japanese Patent Application Laid-Open No. 126288/1979, at the first stage, an organic compound having a solubility in water of less than 10.sup.-2 g/l H.sub.2 O, which functions as a swelling aid, is made absorbed on seed particles, and at the second stage, a slightly water-soluble monomer, the volume of said monomer being about 100 times that of the seed particles, is made absorbed on the seed particles to thereby prepare swollen particles and then the monomer is polymerized by the use of a water-soluble polymerization initiator (e.g., potassium persulfate) or oil-soluble polymerization initiator (e.g., azobisisobutyronitrile) while maintaining the swollen particle form. This method, however, has several disadvantages. For example, when an oil-soluble polymerization initiator is used, monomer droplets remaining unabsorbed on the seed particles are polymerized as such. As a result, a large amount of coagulate is formed and the yield is decreased. When a water-soluble polymerization initiator is used, even if the concentration of the emulsifier is below the critical micell concentration, low molecular weight polymers formed by polymerization in an aqueous phase act as the emulsifiers, thereby allowing so-called soap-free emulsion polymerization to partially or wholly proceed, and thus the form of swollen particles cannot be maintained. Another problem of the above method is that polymer particles obtained are not in the spherical form but in the deformed form by the action of the organic compound of low water solubility as a swelling aid to be absorbed on the seed particles at the first stage.
In order to overcome the above problems, a method of swelling seed particles without the use of a swelling aid has been developed (see J. H. Jansson, M. C. Wellons & G. W. Poehlein, J. Polym. Sci., Polym. Lett. Ed., 21, 937-943 (1983)). In accordance with this method, a monomer and an oil-soluble polymerization initiator are mixed finely dispersed in an aqueous medium to prepare an aqueous dispersion, and this aqueous dispersion is added to a seed particle dispersion (latex), whereby the seed particles are swollen in a high swelling ratio. This method, however, has several disadvantages and is not sufficiently satisfactory for practical use.
For example, when styrene is used as the monomer and added in such an amount that the weight ratio of monomer to seed particles is 100:1, even if seed particles having a uniform diameter are used, only swollen particles having a diameter of about 1 to 3 .mu.m can be obtained while maintaining the uniform diameter. If swollen particles having a diameter exceeding the above range are intended to prepare, the particle diameter distribution is inevitably broadened; monodisperse, large-sized polymer particles having a diameter of more than 3 .mu.m cannot be obtained. In the case of monomers having a high water solubility, such as methyl methacrylate (MMA), swollen particles having a diameter range of about 1 to 3 .mu.m are difficult to prepare while maintaining the uniform particle diameter. Even if uniform particles are obtained, their stability is poor; the uniformity is quickly lost and the particles become uneven in diameter. Moreover, in the case of monomers having a low water solubility, such as 2-ethylhexyl acrylate, monomer droplets in the aqueous dispersion are stable and remain unabsorbed on the seed particles for a long period of time, thereby preventing the formation of swollen particles having a uniform particle diameter.
In summary, the method disclosed by J. H. Jansson et al. has disadvantages, for example, in that in preparation of large-sized, monodisperse polymer particles by seeded polymerization, only limited monomers such as styrene can be used, and even with such monomers, monodisperse polymer particles having a particle diameter as large as more than 3 .mu.m cannot be obtained.